In an integrated electronic circuit made on the surface of a substrate, it is often necessary to electrically connect together two conducting tracks placed in superimposed parallel planes lying above the surface of the substrate. To do this, the tracks are placed so that their respective projections onto the substrate surface have common portions, especially when the tracks cross over each other. A conducting connection is then placed between the two tracks, which provides an electrical connection between the two tracks in the direction perpendicular to the substrate surface.
Such a connection is known by the term “via”, in the jargon of a person skilled in the art. A “via” comprises a substantially cylindrical part with an axis perpendicular to the substrate surface, which is in contact with each of the two tracks. This cylindrical part is particularly small, with a section of about 0.1 micrometres at the interface with each track, parallel to the substrate surface, and about 0.5 micrometres along the direction perpendicular to the substrate surface. These dimensions are associated with the method of producing the connection.
The conducting tracks are made of a metal, such as copper or aluminium for example. The density of these tracks is often less than the theoretical density of the same bulk material, so that the tracks may be subject to contraction of their constituent material, a contraction especially caused by thermal phenomena, in particular by heating. Such phenomena occur especially during fabrication of the electric circuit or subsequently, during use of the circuit. This contraction may cause a hole to form in the track. Typically, such a hole is formed at the interface between the connection and the track. The electrical continuity between the two tracks may then be broken, which may prevent the correct operation of the electric circuit. In some cases, the electrical continuity is not broken, but the electrical resistance between the connection and the track is increased by the presence of the hole. The reliability of the electrical circuit is then reduced, especially due to electromigration effects.
More specifically, after producing the tracks, the material of the tracks has a porosity, such that elementary voids are present distributed throughout the material. These elementary voids may in particular be porosity holes, or else structural vacancies in the material on an atomic scale. Elementary voids may also be generated by stresses in the material of the tracks, which result from differential expansions between the material of the tracks and a material outside the tracks, especially an insulating material within which the tracks are placed. Such elementary voids especially appear when the material of the tracks and the insulating material have different respective thermal expansion coefficients. Other types of elementary voids may be present, but their nature and their origin can sometimes be difficult to identify.
It has been observed that the elementary voids have a certain mobility within the conducting material of the tracks, and especially that they move in order to come together at preferential sites, forming a larger void at this location. Sites at which elementary voids of one of the tracks come together are, for example, junctions between the grains of the track material, or locations on the surface of this track which have particularly high local stresses. This is particularly the case at an edge belonging to an interface between a track and a material surrounding the track, which is harder or more elastic than the material of the track itself, or at a smaller interface between the track and a material outside the track, different from the material surrounding the track.
The interface between a connection and a track is therefore a favoured site for elementary voids to come together. Consequently, a void of dimensions comparable to the section of the connection may appear at this interface. When a large number of elementary voids are involved in the formation of a localized void at the interface with the connection, the electrical contact between the connection and the track may be broken. This loss of electrical contact takes place all the more quickly the smaller the connection, especially for electronic circuits with a high integration level, and all the more quickly when the track is wide, and therefore likely to contain a large number of elementary voids.